High purity and cleanness is required in those semiconductor material gases, purge gases, carrier gases, etc. which are employed in the semiconductor manufacturing process. Presence of contaminant such as particles, oxygen and moisture if any in these gases causes problems such as insufficient device characteristics attributed to oxidation or metal pollution and reduction in the yield of products.
Generally, when a gas cylinder valve is set to a plant for supplying or producing a semiconductor process gas, the atmospheric air migrates between a mouth ring (or port) of the valve and a pipe for fitting to such a plant, since the port of the gas cylinder valve is exposed to the atmospheric air. The thus migrated atmospheric air is removed by purging with an inert gas such as a nitrogen gas and an argon gas or by evacuation. However, if the migrated contaminant is removed insufficiently, residual contaminant migrates into the gas cylinder during manufacture of a gas. For example, in the case of a gas having reactivity with the atmospheric component gases such as oxygen and water, these gases are causative of change in the concentration of the product gas with time, oxidation reactions to form impurity products and corrosion on the metal surfaces of the gas cylinder and the gas cylinder valve to be brought into contact with the gas.
Further, in supplying a gas, contamination with the residual impurity contents occurs from the gas supply plant to a gas consuming plant. The influence of contamination during the supply of the gas appears not only in the gas system but also as reduction in the yield of products and deterioration in the electric characteristics.
Gas cylinder valves employed conventionally most frequently are of the single-ported structure in which one port serves both as a gas outlet port and a gas inlet port, and the valves contain dead spaces, so that it takes much time for removing the atmospheric air components migrated into the gas cylinder valves when they are attached to gas cylinders.
Meanwhile, gas cylinder valves are proposed as described in Japanese Unexamined Patent Publication Nos. 106749/1993 and 281026/1994 so as to overcome the above problem. Although these valves can remove contaminant contained therein quickly, they involve problems in terms of safety in that the valves themselves have very large dimensions due to the block valve structure having two gas cylinder valves integrated into one block causing gas cylinders having such valves to tumble easily during transportation. These gas cylinder valves involve further problems in that the cost increases on a wide margin due to their intricate structures compared with the ordinary cylinder valves and that an extra piping for a purge gas must be provided for the gas plant side. While there is also known a gas cylinder valve incorporated with a purge valve, this gas cylinder valve has an intricate valve structure and a relatively large size and is also expensive.